This invention relates to the separation of isotopes, and more particularly, it relates to isotope separation methods and apparatus utilizing energy state selectivity.
In the past many schemes for the separation, or at least the enrichment, of particular isotopic forms of certain chemical elements have been proposed and demonstrated including separation by gaseous diffusion through a porous barrier, electromagnetic separation using a mass spectrometer, centrifugal separation, separation by thermal diffusion, separation by fractional distillation, electrolytic separation, and chemical separation using isotopic exchange reactions with other elements.
Recently, several isotope separation schemes have been devised based on selective optical excitation of a desired isotope in a mixture of isotopes using a tunable laser. In these schemes the laser is tuned so that its output coincides in frequency with an allowed transition of the desired isotope but not with that of the undesired isotope. The selectively excited isotope is subsequently ionized by either absorption of light (supplied from a second laser or an incoherent source), by contact with a heated ionizing surface, or by generation of an electric discharge. Ions are thus produced from the excited isotope only, and these ions are then physically separated from the mixture.
A practical problem encountered with many of the aforementioned isotope separation techniques, e.g., the gaseous diffusion method, is that the enrichment ratio of the isotopes being processed (i.e., the ratio of the percentage of the desired isotope in the output mixture to the percentage of the desired isotope in the input mixture) is quite low. As a result, an extremely large number of stages are needed to obtain useful levels of enrichment of the desired isotope. Other of the aforementioned isotope separation techniques, e.g., electromagnetic separation, provide substantially higher enrichment ratios than the gaseous diffusion process. However, such high enrichment ratio processes are capable of handling only small amounts of isotopic material and, therefore, are impractical for high volume use. In addition, most of the aforementioned isotope separation techniques require a relatively large amount of energy per separated atom or molecule.
Another field of technology of relevance to the present invention but which heretofore was never associated with isotope separation is that of the molecular beam maser. In the original molecular beam maser a beam of ammonia molecules was formed by allowing ammonia molecules to diffuse out of a directional source consisting of many fine tubes. The beam then traversed a region in which a highly nonuniform electrostatic field formed a selective lens, focusing those molecules which were in upper inversion states while defocusing those in lower inversion states. The upper inversion state molecules emerging from the focusing field were directed into a resonant cavity in which downward transitions to the lower inversion states were induced. For further details concerning the molecular beam maser, reference may be made to the paper by J. P. Gordon, H. J. Zeiger and C. H. Townes, "The Maser--New Type of Microwave Amplifier, Frequency Standard, and Spectrometer", Physical Review, Vol. 99, No. 4 (Aug. 15, 1955), pages 1264-1274, and to U.S. Pat. No. 2,879,439, issued Mar. 24, 1959 to C. H. Townes and entitled "Production of Electromagnetic Energy."
However, prior to the present invention it was not seen how molecular beam maser technology could be modified and extended to advance the state of the isotope separation art.